During the 1970s and 1980s, the U.S. Department of Defense developed and fielded a highly accurate, globally usable radio-positioning system. It was designed to be used for self-location of military systems on land, in the air or in low earth orbits, or on the surface of bodies of water. This system, which has been known as NAVSTAR and as GPS (Global Positioning System), is now in wide use by both military and commercial vehicles, air and spacecraft, missiles, foot soldiers, and civilians engaged in business and pleasure. Each receiver listens to signals from the visible members of a constellation of transmitter satellites launched by the U.S. Department of Defense into three approximately 11000-mile near-circular orbits around the earth. FIG. 1 (prior art) is a simplified functional diagram of the GPS satellite payload.
Application of GPS requires that a receiver receive signals from at least three different satellites and measure with great accuracy the time-differences between their signals. Then, with precise knowledge of the satellites' orbital positions and any offsets in their clock-timed signals extracted from the received signals, the receiver performs spherical-geometry computations to determine its own location with respect to the earth. Received signals from three satellites are needed to determine horizontal position on the earth's surface, while four signals will allow determination of both surface coordinates and altitude. In practice, many more satellites are required to provide service continuously over most of the earth's surface. Because the satellites orbit at about half the geo-synchronous altitude, the set of 3, 4, or more satellites used for location calculation must continually change. Accuracy depends on accurate quantum clock timers on each satellite, and on the geometry of the measurement. Maximal accuracy is obtained if three satellites are spaced 120 degrees apart on the horizon, with a fourth directly overhead, but near-maximal accuracy is obtained almost continuously within the temperate zone.
All transmitter satellites share narrow (10 MHz) allocated electromagnetic bands, centered at 1575.42 MHz and 1227.6 MHz. These are referred to as the L1 and L2 bands respectively. In order to share these bands and be separately receivable, pulse code modulation (PCM) is used. Each satellite's signals are distinguished by a separate pseudo-noise code sequence, and each is detected in a receiver by generating a corresponding code sequence in the receiver and demodulating the signal through its use. FIG. 2 (prior art) is a functional block diagram of a user system configuration with separate receivers and position estimating functions.
As shown in FIG. 1, each GPS satellite broadcasts two signals in the L1 band and one in the L2 band. The Clear/Acquisition (C/A) signal, currently broadcast only in the L1 band, repeats every millisecond, has pulse (chip) rate of 1.023 MHz, is intended for easy acquisition by receivers, and provides accuracy of 100 meters in horizontal location. The Precision (P) signal repeats weekly and has a chip rate of 10.23 MHz, providing basic horizontal accuracy of about 30 meters. Accuracy can be improved by extended measurements. The entire satellite constellation thus operates using code-division multiple access (CDMA) to enable separation of signals from all satellites.
Receivers are, with the exception of necessary radio-frequency elements, largely comprised of digital computers. Proper use of multiple computers permits simultaneity of performance of some of the many necessary computation functions, and generally improves dynamic accuracy.
Because of the electrically charged ionosphere surrounding the earth--well below the altitude of the GPS satellites--the travel time of electromagnetic waves reaching earth-based receivers is slightly altered from that assuming free-space paths. Correcting this small error is required only for certain military applications, and provided for in GPS by transmitting a simultaneous P signal on the L2 carrier.
The designers of GPS provided the C/A signal for two purposes: to enable acquisition of the more precise P signal by military receivers, and also for an "in the clear" low-accuracy signal for civilian use. The P signal could, according to that design concept, be replaced in times of national emergency by a similar but differently coded Y signal known only to friendly military. Military receivers would be able to acquire the P/Y signal without the help of the C/A signal. However, since no provision was made for disabling the C/A signal, a potential enemy could use it in wartime. With the original signal design, jamming would affect both C/A and Y signals and would not only deny the C/A signal to hostiles but would also deny the Y signal--and use of GPS--to friendly forces.
For a more detailed discussion of GPS see the text "Global Positioning System: Theory and Applications--Volume 1", B. W. Parkinson and J. J. Spilker, Jr., PROGRESS IN ASTRONAUTICS AND AERONAUTICS, Vol. 163, 1996.